Permanent magnet generators represent a simple and reliable form of generator construction that is suitable for use in situations where high reliability is paramount. In essence, and as schematically illustrated in FIG. 1, a rotor 2 carries a plurality of permanent magnets 4, 6 and 8 regularly disposed around its circumference. Rotation of the rotor causes the magnets to be moved towards and then away from the stator coils 10, of which only one is shown, held by the stator of the generator.
The voltage generated in each stator coil is
  V  =            -      N        ⁢                  ⅆ        Φ                    ⅆ        t            where                Φ is the instantaneous value of the magnetic flux cutting the stator coil.        N is the number of turns of the stator coil.        
This shows that, at least while no current is being drawn, the generator output voltage is proportional to the rotor speed.
When a current is drawn the situation becomes a little more complicated because:                i. the coils have a finite resistance and consequently part of the voltage is dropped across the resistance of the coil.        ii. current flow within the coil causes it to produce its own magnetic field, which then interacts with the field from the permanent magnets causing the voltage to change.        
Furthermore it is known that when uncontrolled, the generator output voltage can vary over a wide range dependent upon generator speed and load.
It is inconvenient for the voltage supplied to a load to vary over a wide range and therefore some form of stabilisation is useful.
It is known to convert the generator output to a DC voltage using a power converter. A controller can be used to control the power converter so as to provide a desired output voltage at a DC link.
Generally, in order to perform stable control of the generator, it has been necessary to include a position detector on the rotor. Sensing the rotor position allows the voltage waveform under no load conditions to be inferred from a knowledge of the generator design and sensor position. This can be compared with the current waveform to obtain a measurement of current angle. Such a system is described in U.S. Pat. No. 6,583,995.
U.S. Pat. No. 5,177,677 includes means (not described in detail) for measuring the “source voltage” of a generator, as described at column 6 lines 1 to 3. This measurement of source voltage is probably made with an additional sense coil wound onto the stator such that an output voltage under no-load conditions can be inferred. The circuit also measures the current waveforms and consequently the phase shift between the voltage and the current can be directly derived to give the power factor.
U.S. Pat. No. 6,239,581 discloses a regulation circuit which monitors the voltage occurring across a load and on the basis of this measurement may pass current through an inductor connected in parallel with the generator so as to add a “lag” into the power factor.